Cellular Division: Mitosis and Meiosis Lecture Slides PDF

Cellular Division: Mitosis and Meiosis Dr. Praveen Babu Life Science for Engineers BIO 213  Cells Divide to Maintain Life Cell division is essential and produces a continuous supply of replacement cells Growth and development required cell divisi...

Cellular Division: Mitosis and Meiosis Dr. Praveen Babu Life Science for Engineers BIO 213  Cells Divide to Maintain Life Cell division is essential and produces a continuous supply of replacement cells Growth and development required cell division Reproduction requires cell division too Organisms must reproduce to propagate their species and pass genetic information down to offspring Asexual reproduction: most straightforward & ancient Genetic material is replicated; cell splits into two Generates genetically identical offspring (i.e., clones)  Cells Divide to Maintain Life Sexual reproduction: offspring’s genetic makeup is derived from two parents Each parent contributes a sex cell or gamete Traits mixed up/recombined so genetically different offspring are produced Upon fertilization, a zygote forms, grows, and divides mitotically or “clones” itself into the trillions of cells in our body  Cells Divide to Maintain Life Sexual reproduction: offspring’s genetic makeup is derived from two parents Each parent contributes a sex cell or gamete Traits mixed up/recombined so genetically different offspring are produced Upon fertilization, a zygote forms, grows, and divides mitotically or “clones” itself into the trillions of cells in our body Cell Division: Meiosis to create unique gametes; mitosis to simply regenerate a cell Both require DNA to be replicated first  Cell Death is a Part of Life Cells also die in predictable ways Apoptosis: programed cell death Both cell division and apoptosis work in concert to shape new structures during development Example: apoptosis removes dead or decaying cells from the body (e.g., skin cells that “peel” because of sun burn)  Precedes Cell All organisms must replicate their DNA before they can Division reproduce As a cell splits in two, each “daughter” cell must receive a new copy of its genome (i.e., all chromosomes) DNA is replicated semi-conservatively  Precedes Cell DNA is replicated semi-conservatively Division Helicases unwind DNA at origins of replication DNA polymerase builds new DNA (5’  3’) off of each template strand Ligases seal up adjacent DNA molecules DNA replication is incredibly accurate because of proofreading and error repair mechanisms Only 1 in a billion nucleotides are incorrect! Mutations due occur, however Some of the worst ones are mutations to proofreading or repair enzymes (e.g., BRCA1/2 can be cancerous)  Bacteria/Archaea Divide by Binary Fission In prokaryotes, asexual reproduction occurs by binary fission  Eukaryotic Cell Division In eukaryotes, distributing DNA into daughter cells requires a choreographed sequence of events DNA is divided among many chromosomes Chromosomes are made of chromatin (DNA + histone proteins) 6 ft of DNA is packed into each of our nuclei via packing; chromatin is wound up tight into fibers of nucleosomes  Eukaryotic Cell Division In eukaryotes, distributing DNA into daughter cells requires a choreo- graphed sequence of events DNA is divided among many chromosomes Chromosomes are made of chromatin (DNA + histone proteins) 6 ft of DNA is packed into each of our nuclei via packing; chromatin is wound up tight into fibers of nucleosomes  Eukaryotic Cell Division In eukaryotes, distributing DNA into daughter cells requires a choreo- graphed sequence of events DNA is divided among many chromosomes Chromosomes are made of chromatin (DNA + histone proteins) 6 ft of DNA is packed into each of our nuclei via packing; chromatin is wound up tight into fibers of nucleosomes After replication and only before division do chromosomes condense  Eukaryotic Cell Division After replication and only before division do chromosomes condense Condensed replicated chromosomes have two sister chromatids and a visible centromere Humans are diploid (2n); we have two copies of each chromosome 23 pairs of chromosomes total 22 pairs of autosomes 1 pair of sex chromosomes (designated X and Y chromo’s) Two members of a pair make up a homologous pair of chromosomes  Eukaryotic Cell Division Two members of a pair make up a homologous pair of chromosomes Homologous because they contain the same genes, but may have different alleles or “flavors” of the same gene  The Cell Cycle Our cells alternate between two states: division and nondivision Cell cycle: the sequence of events between divisions A cycle consists of: Interphase – the time between divisions; cell growth and DNA replication Mitosis – division of the chromosomes Cytokinesis – division of the cytoplasm; splitting into 2 cells  The Cell Cycle - Interphase Interphase occurs just after cells have divided; cells must grow and mature through three stages: G1 (Gap 1) – RNA, proteins and other molecules are made S (synthesis) – DNA is replicated G2 (Gap 2) – Preparing for cell division Keep in mind that not all cells cycle; some arrest in G1 or permanently arrest in G0 and never divide  The Cell Cycle - Mitosis Mitosis prepares cells for division by distributing DNA Replicated chromosomes (46 x 2) must be given to two daughter cells accurately (46 each) for proper cellular function Mitosis or “M Phase” occurs in six arbitrary phases: Prophase (Prometaphase) Metaphase Anaphase Telophase Cytokinesis  The Cell Cycle - Mitosis Prophase/Prometaphase: chrom’s condense into two longitudinal strands or chromatids; nuclear envelope breaks down; spindle fibers (microtubules) attach Chromatid strands are held together by a centromere Because chromosomes were replicated during interphase, similar chromatids are joined at a centromere forming sister chromatids  The Cell Cycle - Mitosis Metaphase: chromosomes with spindle fibers attached to their centromeres move to the middle of the cell (the metaphase plate or spindle equator) Anaphase: centromeres divide to convert each sister chroma- tid into a chromosome  Mitosis & Cytokinesis Telophase: chromosomes reach opposite ends of the cell and decondense; nuclear envelopes reform; cells start to take shape Cytokinesis: cleavage furrow forms, deepens, and cytoplasm divides to yield two daughter cells Daughter cells are each diploid (2n); have two copies of each chr’some (23 pairs, 46 total); genetically identical  The Cell Cycle - Mitosis Mitosis describes what is happening in somatic cells Mitosis vs. Meiosis Mitosis describes what is happening in somatic cells Meiosis in germ cells is the form of cell division that yields gametes (sex cells; e.g., eggs and sperm) Members of a chromosome pair separate from each other so the gametes produced are haploid (n) Haploid gametes have one copy of each chromosome (0 pairs, 23 total); NOT genetically identical Fertilization is the fusion of gametes (an egg and sperm cell) and restores the number of chromosomes back to 23 pairs, 46 total (2n) Mitosis vs. Meiosis Mitosis describes what is happening in somatic cells Meiosis in germ cells is the form of cell division that yields gametes (sex cells; e.g., eggs and sperm) Members of a chromosome pair separate from each other so the gametes produced are haploid (n) Haploid gametes have one copy of each chromosome (0 pairs, 23 total); NOT genetically identical Fertilization is the fusion of gametes (an egg and sperm cell) and restores the number of chromosomes back to 23 pairs, 46 total (2n) Fertilized egg is a diploid (2n) zygote and can develop properly  Chromosome In meiosis, diploid cells (2n) undergo replication and Number then two rounds of division yielding haploid (n) gametes Chrom.’s associate into homologous pairs Crossover events can occur via homologous recombination Chrom’s line up in either orientation Both increase genetic diversity in offspring NOTE: After Telophase I / Interkinesis all chromosomes are still in a duplicated state, but cells are haploid (n) Meiosis II Begins with Haploid Cells In meiosis, diploid cells (2n) undergo replication and then two rounds of division yielding haploid (n) gametes  Meiosis II Begins with Haploid Cells 0:38 Meiosis II Begins with Haploid Cells  Meiosis Increases Genetic Diversity Meiosis increases genetic diversity via crossing over during prophase I, independent assortment during metaphase I, as well as random fertilization Thus, one couple could create more than 70 trillion genetically unique individuals Mitosis vs. Meiosis  Sometimes Occur in Meiosis Polyploidy results in extra chromosomal sets; fatal in humans, important to plant evolution  Sometimes Occur in Meiosis Nondisjunction events result in gametes with one extra or missing chromosome Sister chromatids fail to separate at 1st or 2nd meiotic division Down syndrome or trisomy 21 is the most common example of extra autosomes  Sometimes Occur in Meiosis Nondisjunction events result in gametes with one extra or missing chromosome Sister chromatids fail to separate at 1st or 2nd meiotic division Down syndrome or trisomy 21 is the most common example of extra autosomes Sex chromosome abnormalities are also common